1. Field of the Invention
This invention relates to a solid state image pickup device, and more particularly to a stacking type solid state image pickup device in which the light detecting sections are formed on a scanning circuit.
2. Description of the Prior Art
Conventional solid state image pickup devices consist of light detecting sections such as photodiodes arranged in matrix form and a scanning circuit for sequentially selecting the signals detected by these light detecting sections. For example, a solid state image pickup device constituted by a combination of a light detecting matrix with a field effect transistor circuit for XY scanning (hereinafter referred to as the XY matrix type) is disclosed in Japanese Patent Publication No. 45(1970)-30768. Further, a solid state image pickup device formed by a combination of a light detecting matrix with a bucket brigade device (BBD), a charge coupled device (CCD) or a charge transfer section of charge priming transfer (CPT) type is described for example in Japanese unexamined Patent Publication Nos. 46(1971)-1221and 47(1972)-26091, and the Japanese magazine "Denshi Zairyo" (Electronic Materials), March 1980, page 6 et seq. However, these known solid state image pickup devices are disadvantageous in that the light use efficiency per unit area of the device is extremely low because the light detecting sections and the circuit for sequentially selecting the signals detected by the light detecting sections (including the aforesaid XY matrix circuit, charge transfer circuit, the field effect transistors serving as switching elements for feeding charges to these circuits, and the like) are positioned two-dimensionally on the same plane.
Recently, it has been proposed to improve the light use efficiency by stacking photoconductive materials on the aforesaid scanning circuit so as to form a multilayer solid state image pickup device. For example, as disclosed in Japanese unexamined Patent Publication No. 49(1974)-91116, photoconductive materials are stacked on an XY matrix type scanning circuit consisting of field effect transistors. In Japanese unexamined Patent Publication No. 55(1980)-27772, it is disclosed to form polycrystalline deposited films by use of heterojunctions of semiconductors comprising the compounds of groups II to VI of the periodic table on a BBD type or CCD type scanning circuit.
On the other hand, an attempt has been made to use amorphous silicon to form semiconductors for solar batteries or electrophotographic photoreceptors. The term "amorphous silicon" as used herein means silicon which, unlike crystalline silicon exhibiting a long-period atomic configuration, exhibits no periodic characteristics of atomic configuration. Conventional amorphous silicon exhibited very unsatisfactory photoelectric characteristics because of structural defects due to lack of periodic configuration. More recently, however, it was found that amorphous silicon containing an element such as hydrogen or fluorine, which reduces the gap state of electrons and positive holes, or reduces the localized states, in the energy gap, exhibits a high photoconductivity at a relatively high resistivity (10.sup.8 to 10.sup.9 .OMEGA..cm). In other words, the amorphous silicon contains elements which reduce the localized states in the energy gap to make the same approximate, more closely, the intrinsic crystalline silicon. More importantly, it was further found that, as in the case of crystalline silicon, the conductivity of such amorphous silicon can be controlled by doping it with an impurity, as described by W. E. Spear and P. G. Comber in "Solid State Communication", Vol. 17 (1975), page 1193 et seq. Thus amorphous silicon has attracted much attention in the basic research and application fields. For example, application thereof to photoelectromotive force devices is described by D. E. Carlson and C. R. Wronski in "Applied Physics Letters", Vol. 28(1976), page 671 et seq.
Under the above circumstances, it has been proposed as disclosed in Japanese unexamined Patent Publication No. 55(1980)-39404 to use amorphous silicon as the photoconductive material for the above-mentioned multi-layer solid state image pickup device. In this type of solid state image pickup device using amorphous silicon, a single amorphous silicon layer is electrically connected to a source electrode or a drain electrode of the field effect transistor of the XY matrix type or charge transfer type scanning circuit combined with MOS type field effect transistors positioned in the matrix form, and a transparent electrode is formed on the amorphous silicon layer.
The inventors eagerly studied the above-described conventional stacking type solid state image pickup device comprising light detecting layers of amorphous materials mainly consisting of silicon superposed on a scanning circuit, and found that conventional hydrogenated amorphous silicon obtained by glow-discharging chemical vapor comprising silicon hydride gas, hydrogen gas and the like exhibits low sensitivities to blue light and red light and cannot give sufficient photoconductive characteristics over the whole visible light region. This presents a very real problem particularly with regard to artificial light such as tungsten light having a high red light intensity. Further, it was found that blue light is absorbed by the transparent electrode positioned on the light inlet side of each light detecting section, and the sensitivity of the light detecting section to blue light further decreases. These problems could be solved by amplifying the light signals of wavelengths to which the light detecting section exhibits low sensitivities. With this method, however, noise is also amplified, resulting in a low signal-to-noise ratio. The above problems could also be solved by lowering the sensitivities to other wavelengths to match the light signal of the wavelength corresponding to the lowest sensitivity of the light detecting section. In this case, however, the sensitivity of the whole image pickup device is limited to the lowest sensitivity of the light detecting section.